A short circuit condition in an electrical device can cause an excessive amount of current to be drawn through a power transistor, which may potentially damage the power transistor and other electrical components of the device. Therefore, many electrical devices include a current-limit circuit to limit the amount of current drawn though the power transistor in the event of a short circuit condition. Known current-limit circuits utilize various techniques to provide current limiting protection. Due to the extensive prior use of bipolar transistors, many conventional current-limit circuits are designed for bipolar circuits, which may not be suitable for metal oxide semiconductor (MOS) circuits.
U.S. Pat. No. 4,612,497 to Ulmer describes a current-limit circuit that utilizes MOS technology. The current-limit circuit of Ulmer includes a P-channel metal oxide semiconductor (PMOS) driver transistor. The source of the driver transistor is connected to a supply voltage, while the drain of the driver transistor is coupled to a current mirror. The current mirror is formed of a pair of PMOS transistors. The sources of the current mirror transistors are coupled to the driver transistor. The drain of the first current mirror transistor is connected to an output terminal, while the drain of the second current mirror transistor is connected to electrical ground via a current sink transistor. The drain of the second mirror transistor is also connected to the gates of the first and second mirror transistors. The first current mirror transistor is a power device and is significantly larger than the second current mirror transistor and the current sink transistor. During normal operating conditions, the first mirror current transistor operates in a non-saturated region and the amount of current sourced by the first mirror current transistor to the output terminal is not limited. However, when the first mirror current transistor becomes saturated by increased supply of current beyond a threshold value, the current sourced by the first current mirror transistor is limited, due to the current mirror effect.
A concern with the current-limit circuit of Ulmer is that current is consistently drawn through the second current mirror transistor and the current sink transistor, even during normal operating conditions. Thus current-limit circuit of Ulmer is implemented in a portable electrical device, power dissipation may become an issue.
Although known current-limit circuits, including the current-limit circuit of Ulmer, operate well for their intended purpose, what is needed is a current-limit circuit that provides short-circuit protection without significant power dissipation during normal operating conditions.